Brooklyn’s grandest public space at the top of Prospect Park has always been a work in progress. Grand Army Plaza, an oval-shaped public space composed of monuments ringed by an inner and an outer roadway, was built as the main entrance to the park in 1866, serving as a buffer between nature and city and happened to be the confluence of some of Brooklyn’s busiest avenues. Over the years, a monumental archway was added, fountains came and went, and eventually the roads were widened until the lush plaza was effectively cut off from the surrounding Prospect Heights and Park Slope neighborhoods. Last week, however, after months of construction to tame the out-of-control roadways, a group of civic leaders and officials gathered in what was once a busy street to celebrate the newly reclaimed plaza.